Linear motion systems are used to accurately position semiconductor wafers during commercial semiconductor fabrication processes and automated assembly of semiconductor devices. One form of a linear motor offering high speed and accuracy is a Sawyer motor.
In a typical semiconductor processing application, a Sawyer motor operates on a steel waffle platen and is floated by a thin layer of air (about 8-13 microns thick) by an air bearing. The Sawyer motor is used to position a semiconductor wafer so that, for example, the die composing the wafer may be tested before packaging. With low mass loads, Sawyer motors are capable of high position resolution (e.g., 2 microns with open loop micro-step control), rapid acceleration (e.g., 1 g), and high speed movement (e.g., 1.5 m/sec).
In many applications, Sawyer motors that are built to identical specifications actually vary in their performance characteristics. A common cause of these variations is due to errors or variations in the manufacture of these motors. Manufacturing variations include variations in magnetic materials used in fabricating Sawyer motors, and manufacturing variations which cause inaccuracies in motor geometry. If manufacturing variations are large enough, a Sawyer motor may perform out of nominal specification. Since Sawyer motors operate mostly in an open-loop mode, use of motors which perform out of nominal operating specification in a linear positioning system can lead to positional inaccuracies in the system. For example, when a single Sawyer motor that is out of specification, travels in a linear direction (e.g., an X direction), it may travel an inaccurate distance for any given drive current. At the end of a full drive cycle, the motor may have traveled the correct total distance. However, during the drive cycle the motor may not have been at the correct linear position at any given instant of time within the drive cycle. If the motor is stopped at a partial cycle, the motor's position may deviate from the intended position. This type of inaccuracy is called a translational inaccuracy.
Translational inaccuracies can manifest themselves as rotational errors when two Sawyer motors are configured in a side-by-side assembly, and the assembly is made to travel in some linear direction. For example, if the two Sawyer motors of the above assembly have mismatched performance characteristics relative to each other, and are driven by the same current source, one motor will tend to travel a greater distance than the other. As a result, the assembly will tend to rotate during a current drive cycle (typically one sinusoidal period), about the Z axis perpendicular to the XY plane of linear motion.
Given the above motor assembly, if the rotation angle were zero at the beginning of a full drive cycle, then at the end of a full drive cycle the motor assembly may again be at a zero rotation angle. However, during the drive cycle, if the motor assembly is stopped at some partial cycle the motor assembly may have rotated some absolute angle about the Z axis. Such a rotational angle about the Z axis is known as a yaw angle.
If, however, both Sawyer motors in the motor assembly are identical and without an impedance mismatch, then they will move in a straight line on the XY plane during the drive cycle. Consequently, the motor assembly will not experience a significant yaw rotation error at any time during the drive cycle.
Present methods of correcting for positional errors in Sawyer motors that are used in a side-by-side motor assembly often requires reworking the Sawyer motor itself by regrinding the motor's pole face surface. In general, regrinding Sawyer motors is both time consuming and costly. Furthermore, the regrinding process often results in low yields of Sawyer motors corrected to operate within performance parameters. Consequently, in many precision applications requiring tight tolerances, a significant percentage of Sawyer motors may be scrapped as unusable.